Fibers have been added to organic and inorganic matrices to improve the viscosity and the handling characteristics of the mix and the toughness of the cured or solidified product. Fibers provide a combination of viscosity control and reinforcement, whereas particulate and chemical additives only improve viscosity. Asbestos fibers and other inorganic fibers, such as ceramic, glass, and the like, have been used in various resin systems to prepare sealants, mastics, putties and adhesive formulations. Health hazards in the use of asbestos are well known. Moreover, inorganic fibers, because of their intrinsic brittleness, can fracture during mixing to create a higher percentage of very short fibers, and these result in variability and loss of performance as fiber thixotropes, because short, broken fibers generally do not provide maximum reinforcement and viscosity enhancement.
The inorganic thixotropes have been supplanted to a significant extent by polymeric fibers, especially by fibrillated polyolefin and polyaramid fibers. Fibrillation is a means of texturizing which comprises stretching a fiber or film to fully or partially orient it and then breaking it transversely apart or mechanically abrading it to provide an interlaced fibrous structure. Such fibers induce high viscosities, but they are difficult to disperse in resins, and tend to result in inhomogeneous mixtures. Additionally, their adhesion to the cured resin is generally poor unless special surface treatments are carried out on the fibrillated fibers. Examples of such commercially available fibrillated polymeric fibers adapted for use as physical thixotropes are: KEVLAR.RTM., and ULTRATHIX.RTM., poly para-phenyleneterephtalimide, and PULPLUS.TM., linear polyethylene, both products of the Du Pont Company, Wilmington, Del., USA. Also commercially available are polyethylene fibers sold under the tradenames SHORT STUFF.RTM. and FYBREL.RTM. by Minifibers, Inc., Johnson City, Tenn., USA. The use of KEVLAR.RTM. aramid pulped fibers, in the range of from about 0.1 to 5.0% by weight, for viscosity control in poly(vinyl chloride) plastisols, in silicones, in epoxy resins, and in polyurethanes is described in Research Disclosure No. 29676, April 1987.
Acrylic fibers, e.g., fibers of a co-polymer containing acrylonitrile in a quantity more than 85 wt-%, have good extensibility, which prevents their fragmentation during mixing, and they adhere well to most resin systems because of the polar nitrile groups present in the molecule. These attributes make them desirable materials to consider for use where viscosity enhancement in resin systems is needed while maintaining or increasing the strength of the resulting resin-fiber composite. Modacrylic fibers with more than 35 wt % and generally less than 85 wt % acrylonitrile in the co-polymer provide also good extensibility, resistance to fragmentation during mixing and good adhesion to most resin systems, while also providing other attractive attributes such as fire retardancy. Consequently, they also are desirable materials for the present purpose.
The preparation of acrylic fibers by wet spinning, dry spinning, melt spinning, air gap spinning and flash extrusion are well known to those versed in the art.
The use of acrylic fibers having a diameter of less than 100 micrometers and a length of less than 3 millimeters as matting agents in paints has been described in U.S. Pat. No. 4,927,710. It is stated therein that the fiber aspect ratios must be less than 20. Fiber aspect ratio is defined as the ratio of the fiber length to its diameter (L/D). If fiber aspect ratios are above 20, the patent states that the fibers will bend and scratch the rollers used to process the paint. Typically, the disclosure continues, the concentration of the fibers in such paints are disclosed to be about 20% by weight. High fiber concentrations in the mix are permitted when L/D is 20 or less. However, where L/D values are larger, the viscosity greatly increases. When used in caulking and other building materials, this would produce a paste or dough too viscous to be pumped or sprayed even if thinned out at typically high shear rates. The Research Disclosure, cited above, for example, discloses that aramid pulp (or fibers) produces a dough at 5.0 wt % in a poly(vinyl chloride) plastisol. In any event, an increase in viscosity is not the object of U.S. Pat. No. 4,927,710, and in fact the object is to try to avoid such an increase.
In U.S. Pat. No. 4,820,585, is described the preparation of acrylic fiber bundles with improved dispersibility in mortars, concretes, plasters, thermosetting resins, and the like. Such aggregates are made from staple acrylic fibers having a diameter less than 50 micrometers and a length of more than 3 mm and less than 60 mm by bonding them, in bundles having at least 10 fibers per bundle, with various sizing agents. Dispersibility in organic and inorganic solvents is improved because the sizing agent, a water soluble polymer in the example cited, readily dissolves in water, allowing the fibers to disperse in a portland cement mixture. The fibers are not texturized, but special stretching techniques (at least 8 times stretch) and annealing techniques (heating at 150.degree.-200.degree. C., while stretched, then allowing the fibers to relax) are used in the manufacture of such acrylic fibers to make them strong reinforcing agents. Such heat treatments result in tenacities higher than 50 cN/tex and initial moduli greater than 1000 cN/tex, and these values are typical of fibers twice as strong and twice as stiff as would suffice in the typical fibrous thixotropes.
East German Patent Publication, DD-A1-279491, describes the use of acrylic fibers as physical thixotropes in resin systems. The diameter of the fibers is not specified, but the count number is given as 0.05 to 1.0 Tex, with specific examples of 0.52 Tex and 0.12 Tex. This, for the examples, corresponds to a diameter in the range of 23.8 to 11.4 micrometers. The cut length of the fibers in the claims is greater than 0.1 mm and less than 10 mm. By calculation, the aspect ratio of the fibers used in the examples in the East German Patent Publication ranges from 42 to 263.
The calculations leading to the numbers set forth immediately above are as follows:
Count claimed 0.05-1.0 Tex=7.4-33 micrometers PA1 Length claimed 0.1-10 millimeters=100-10,000 micrometers PA1 Coupling short length with small diameter--14&lt;L/D&lt;303 PA1 Coupling short length with large diameter--3&lt;L/D&lt;1351 PA1 The Examples use: 0.52 Tex/1 mm=L/D=42 and 0.12 Tex/3 mm=L/D=263. PA1 (i) to provide improved viscosity to a matrix polymer, PA1 (ii) to provide a composite of a polymer and the thixotropic agent having greater resistance to slumping than the resistance to slumping of a composite comprising the polymer and a thixotropic agent comprising the corresponding uncrimped, non-texturized fibers, and PA1 (iii) to provide improved resistance to separation of the fiber from the resin, i.e., dewetting. PA1 (a) a resin or a combination of a resin and fillers and/or other conventional additives, and PA1 (b) an effective concentration of a physical thixotrope comprising crimped acrylic fibers with a small diameter, a minimum length in the range or 0.3-3 mm, a minimum aspect ratio (L/D) in the 20-50 range, and having greater than 3%, preferably greater than 5%, and most preferably, greater than 10% crimp.
The fibers of the East German Patent are not texturized, e.g., they are unlike the pulps and fibrillated fibers, mentioned above. In contrast to the U.S. Pat. No. 4,820,585, the East German Patent DD 279491 states that the fibers should not be agglomerated in order to facilitate incorporation into the resin system.
In U.S. Pat. No. 4,866,109 are described cut acrylic fibers and their use to reinforce materials such as plastics, rubbers, paints, cement, tar, petroleum residues, polymeric materials and paints. No mention is made of their use as physical thixotropes. The citation does disclose in Example IX that the fibers have a natural crimp of 3% resulting from normal processing conditions. Example IX of the patent is described to produce an unsatisfactory gasket sheet. By the term "crimp", as used herein, is meant the A.S.T.M. D 123, D-13 definition, that is "the difference in distance between two points on the fiber as it lies in an unstretched condition and the same two points when the fiber is straightened under specific tension, expressed as a percentage of the unstretched length." Mathematically, the percent crimp is expressed as follows: ##EQU1##
The normal processing conditions are taught in Col. 6 of U.S. Pat. No. 4,866,109 to involve drawing and annealing, such as was used to produce the thermooxidatively stabilized fibers of the above-mentioned prior art, especially the East German Patent Publication. In any event, U.S. Pat. No. 4,866,109, suggests in Col. 6, that, if crimping is desired, a suitable apparatus, like a stuffing box, should be employed with the stretched acrylic tows. It is noted that all of the working examples in the patent used fibers chopped from tow having the 3% "normal" crimp, in other words, they were not texturized by crimping.
In summary, the citations above show that, in the present state of the art, fibrous organic thixotropes are known to be suitable replacements for inorganic thixotropes, like asbestos, and they generally comprise pulped polyolefin fibers, pulped aramid fibers, and unpulped heat-modified or chemically modified acrylic fibers. Pulped, or otherwise texturized acrylic fibers having more than the normal 3% crimp have not been described as physical thixotropes, and especially no acrylic fiber thixotropes having a special form of texturizing known in the textile art as crimped fibers, have been described for use as thixotropes.
It has now been discovered, and is the subject of this invention, that producing acrylic fibers by means of a texturizing process which at least induces crimping of greater than 3%, preferably greater than 5%, most preferably greater than about 10%, and especially preferably in the range of from about 30 to about 50% crimp along their length results in an unexpected enhancement in their ability to increase viscosity of resin systems, and at the same time decreases resin drainage and, remarkably improves the sagging resistance. For example, as will be shown later, keeping all other variables constant, crimping above 3%, e.g., 30-50%, alone, will increase the viscosity of a polysulfide resin containing 2% by weight of the respective fibers by a factor ranging from 1.25 to 2.2; crimping above 3% , e.g., 30-50%, alone will reduce the vertical slump of a polysulfide resin composition containing 1.25% by weight of the respective fibers by a factor of 0.1 to 0.5; and crimping above 3%, e.g., 30-50%, alone will increase the resin yield stress in a rheometer by a factor of 2 to 8 and decrease the tendency of resin to separate from the fiber, i.e., dewet, by a factor of 3 to 6, (based on comparing Examples 5-6 and 5A and 6A). Such results are nowhere foreshadowed by the prior art and demonstrate manifest advantages in using fibers prepared in accordance with the present invention as physical thixotropes. It will also be shown that the effectiveness of the fiber as a reinforcement is greater than other inorganic and organic thixotropes.
It is a principal object of the present invention to provide microdenier acrylic fibers, i.e., fibers with a denier less than 1.0, and having greater than 3% crimp to act as a physical thixotrope in adhesives, sealants, coatings, building materials, and the like while maintaining satisfactory mixing, processing and surface characteristics. The working examples hereinafter will show that the fibers act as physical thixotropes by forming an interlocking matrix in the substrate fluid when at rest and orienting parallel to the direction of shear when flowing. The fibers can be unfibrillated or in the form of a pulp. The comparative examples will demonstrate that the fibers provide excellent rheological properties because of crimping to greater than 3%, all other variables being constant. Extensibility (elongation) of elastomeric materials is also increased compared to other fibrous thixotropes. Because of the good UV resistance and low moisture absorption of the fibers, they are well suited for outdoor applications. Also, because of their good hydrocarbon and other chemical resistance they are well suited as fuel tank and chemical tank sealant components.
The thixotropic effect of the fibers can be controlled by changing the fiber architecture, including fiber length and diameter. As will be shown hereinafter, the thixotropic effect of the fibers can be defined in terms of the fiber aspect ratio and the volume fraction, when the aspect ratio exceeds a critical minimum.